The invention relates to methods of varying the bending stiffness or relative bending to torsional stiffness of a shaft member, for example in a piece of sporting equipment such as a golf club or hockey stick. The invention also relates to sporting equipment having variable stiffness shafts.
Golf clubs are currently available in a wide range of bend stiffnesses. It has been found that less advanced golfers tend to prefer a softer shaft, whereby the flexing motion of the shaft can add distance to a driving stroke, by generating a more elevated ball trajectory for lower club head speed at impact. Advanced and professional golfers, on the other hand, tend to generate higher club head speeds during a swing, and find that driving distance increases with a stiffer shaft that provides lower ball trajectory with more ball back spin. These general trends are of course subject to wide personal variation; in addition, personal preferences as to shaft stiffness may vary from day to day and even from hole to hole, and may also be affected by weather conditions. For example, a lower drive is generally preferable when hitting into the wind than when hitting with the wind from behind. It has thus been found that golfers need to carry a large number of clubs to be prepared for these eventualities, and further, that as a golfer""s abilities progress, new clubs are frequently required to accommodate changing shaft stiffness preferences.
Club stiffnesses are measured in terms of the natural frequency of the shaft with a standard weight dummy head. Clubs are available in a wide range of stiffnesses, typically on the order of 250xc2x140 CPM (cycles per minute).
There is a similar wide range of personal variation in preferred stiffness of shafts in other types of sporting equipment, e.g., tennis rackets, ski poles, hockey sticks, baseball bats, fishing poles, hurling sticks, lacrosse sticks, and vaulting poles. In addition to differences from individual to individual, the preferred stiffness may vary from day to day, depending on fatigue and other personal variables, and the requirements of the particular sport.
It has been proposed in U.S. Pat. No. 3,833,223 to Skulking to provide a variable stiffness golf club by providing a set of interchangeable inserts for a single club. While this does allow adjustment of the stiffness of the club, it requires the golfer to carry a large set of inserts, and club stiffness is still restricted to the particular values associated with the inserts, rather than being continuously variable. The nuisance associated with frequent changes of the insert may prevent a golfer from making adjustments between holes which would maximize performance.
There thus still exists a need for a method of varying the bending stiffness of the shaft of a golf club or other piece of sporting equipment which allows easy variation through a continuous range of stiffness values.
In one aspect, the invention comprises a piece of sporting equipment having a variable stiffness hollow shaft. The stiffness is changed by using a variation means to adjust the tension on a tensioning means disposed in the hollow shaft. The tensioning means is attached to the shaft at two points. Thus, applying tension to the tensioning means causes compression in the shaft region between the two points. The tensioning means may, for example, be a wire or cable, or a plurality of wires or cables. Many variation means are possible, including a cam, a pinned retractable piece, a lockable lead screw (which may be adjusted by an external actuator), a pump, a sleeve screw, or a xe2x80x9cset and forgetxe2x80x9d displacement actuator. The shaft may have one or more constraint inserts, which may be made, for example, of low density foam, plastic, or an elastomer. The constraint inserts impede dynamic variation of the tensioning means. They may be held in the shaft by compression fit or by adhesive. The sporting equipment may be, for example, a golf club, a tennis racket, a ski pole, a hockey stick, a baseball bat, a fishing pole, a hurling stick, a lacrosse stick, or a vaulting pole.
In another aspect, the invention comprises a method of varying the bend stiffness of a shaft of a piece of sporting equipment, by applying tension to a tensioning member disposed within the shaft and connected to it. Tension on the tensioning member places the shaft in compression and reduces its bend stiffness. The tension may be adjustable through a continuous range of values. The sporting equipment may be, for example, a golf club, a tennis racket, a ski pole, a hockey stick, a baseball bat, a fishing pole, a hurling stick, a lacrosse stick, or a vaulting pole. The tensioning means may be restrained from dynamic vibration.